PHILADELPHIA (AP) — An investigation is underway to determine what caused a propane tank on a food truck in north Philadelphia to explode into a fireball, critically injuring a mother and daughter and sending at least nine others to the hospital, police said.

The truck, which exploded Tuesday evening outside an auto body shop in the city’s Feltonville neighborhood, was towed away Wednesday for examination by investigators.

Authorities said a 42-year-old woman and her 17-year-old daughter, who were working on the food truck, were in critical condition Wednesday with burns across a substantial portion of their bodies. Also hospitalized were a 27-year-old woman, a 23-year-old man and a 13-year-old girl. Chief Inspector Scott Small said about seven more people were treated and released.

Surveillance video from two businesses near the La Parrillada Chapina food truck captured the explosion, followed by a huge fireball that engulfed the truck and crossed to the other side of the street, Small said.

He said the four-foot propane tank that exploded, one of two that fuels the food truck, was found about 150 feet away in a residential backyard.

A utility pole on the other side of the street caught fire, but the fireball did not reach the homes across the street or businesses next to the truck.

Two cars driving by the explosion were briefly engulfed by flames, Small said, and five people inside were treated at area hospitals.

The city’s fire department, along with police detectives and the bomb squad were investigating.

“We want to ensure that this was an accidental explosion. We want to ensure this wasn’t anything criminal,” Small said. “The bomb squad is investigating to see if there is any foul play. That will be the result of a completed investigation.”

Jane Kim, the owner of a store next to the auto body shop, said that the food truck is parked outside daily. She said that she heard the explosion and was shaken by the incident.